From ICT4D Wiki
Social software refers to applications which facilitate virtual connection and collaboration between people on a network. It is sometimes described succinctly as "connection comes before content."
The term came into more common usage in late 2002, largely due to Clay Shirky who organized a "Social Software Summit" in November 2002, having started to use the term in April 2002. Shirky defines social software as "stuff that gets spammed" 
Some of the following tools or concepts might not be strictly related to social software, but they surely have appeared on my path to social software knowledge. All in all, I had to gather all this stuff in some way.